This invention relates to both an amplifier and to a method of achieving low distortion in an amplifier.
This invention has particular application to audio amplifiers.
There has been considerable human effort into attaining low distortion in amplifiers of many applications at all frequencies. In 1950, the best audio power amplifiers produced distortion of about 0.1% at 1 kHz, and in the 1990s, this was reduced to about 0.001% at 1 kHz, and about 0.02% at 20 kHz, although one manufacturer claims 0.0025% at 20 kHz.
A majority of commercial audio power amplifiers more or less follow standard designs.
Details of some examples of these are given in a review by Douglas Self in a series of articles in xe2x80x9cElectronics World+Wireless Worldxe2x80x9d from August 1993 to January 1994, and also in his book, ISBN 0-7506-2788-3, xe2x80x9cAudio power amplifier design handbook,xe2x80x9d Newness, reprinted 1997/8 and a second edition ISBN 0 7506 4527 X, also Newness, 2000. Another book containing a comprehensive review of amplifiers, is authored by Ben Duncan called xe2x80x9cHigh performance Audio Power Amplifiers,xe2x80x9d Newness ISBN 0 7506 2629 1, 1996, reprinted 1997/8.
There are some exceptions to these designs: A Technics SE-A1 amplifier which is known of in some countries incorporates an A-class output stage supplied by a floating low voltage high current power supply. This power supply is connected to B-class High Voltage Output Stage.
An LT1166 integrated circuit is primarily intended to control quiescent bias feeding output transistors in audio amplifiers. The LT1166 consists of a low gain transconductance differential amplifier (gain of 0.125 mho) with an inverting and a non-inverting input. The circuitry has a local negative feedback path connecting an output of the power output stage to the inverting input of the transconductance amplifier. The input of the output stage is the non-inverting input of the transconductance amplifier. Two local dominant poles for stability are formed by the use of shunt capacitors to ground from the transconductance amplifier""s outputs. This Linear Technology application circuitry promises distortions no less than many currently existing commercial products.
In Journal of Audio Engineering Society, vol. 29, no 1/2, January/February 1981, pages 27-30, M. J. Hawksford, discloses as a mere paper publication a theoretical means of cancelling distortion in any amplifier stage, including the output stage. This is achieved by subtracting the signals feeding the output power transistors inputs from the amplifier output, and then adding this signal back into the signal driving the output transistors"" inputs.
Iwamatsu in U.S. Pat. No. 4,476,442 again as a mere paper publication disclosed circuitry based on the principles of Hawksford. In one embodiment, Iwamatsu discloses floating power supplies supplying the adding and subtracting circuitry. These floating supplies follow a voltage equal to the sum of the output signal plus a signal linearly proportional to the current flowing through the output load. However, Iwamatsu""s circuits do not include local dominant poles.
Robert R. Cordell in xe2x80x9cMOSPOWER APPLICATIONS,xe2x80x9d Siliconix Inc. ISBN 0-930519-0, 1984, 6.6.3 discloses an audio power amplifier essentially the same as one of the Hawksford""s circuits, but including the essential local dominant poles required for stability. This circuit has no provision for thermal stability, nor floating power supply rails, which are rare in amplifiers.
The current inventor Bruce H Candy previously in U.S. Pat. No. 5,892,398 as a mere paper publication only, discloses an amplifier also utilizing the principles of Hawksford, but including local dominant poles required for stability, thermal tracking circuitry for thermal stability, floating power supplies which track the output signal, rather than to the sum of the output signal plus a signal linearly proportional to the current flowing through the output load as in the case of Iwamatsu. Candy also discloses an output stage input current source load which is also supplied by power form the floating power supplies. Candy claims that it is possible with this arrangement to attain a distortion of the order of 1 part per million at 20 kHz at several hundreds of watts output.
Williamson et al. in U.S. Pat. No. 5,396,194 describes as a mere paper publication a switch mode amplifier containing floating low voltage high current power supplies which supply an A-class amplifier. This is similar to the Technics SE-A1 except that the drive circuitry is switch-mode rather than class-B and that the power supplying the A-class amplifier is derived from the switch mode power supply rather than a separate power supply. All the claims are concerned with the switching power saving technique.
In one of the Williamson paper descriptions there was described floating power supplies to supply small signal operational amplifiers which are connected as servo loops to control the current flowing through the output devices. There are two feedback paths containing a capacitor which form two local dominant poles which are essential for stability.
The current inventor Bruce H Candy has considered an amplifier consisting of at least one operational amplifier, a first error correction amplifier, connected up as a servo loop to control the output voltage, as opposed to the output current as in the case of Williamson et al. These operational amplifiers would be supplied by power from floating power supplies which track the output voltage.
Candy further has considered a local dominant pole being required for stability, and the advantages of using wide-band operational amplifiers, with gain bandwidth products of more than 100 MHz. In addition, Candy has considered a second error correction amplifier, consisting of another operational amplifier, also preferably wide-band, connected up as a servo loop to control the output voltage stage which includes the first error correction amplifier. In other words, Candy has considered a 2nd order local dominant pole formed by the signal path being amplified by two error correction stages in series.
This also would be supplied by the floating power supplies. Further considered are the advantages of implementing high gain stages with local negative feedback and the attendant local dominant poles required for stability in other stages of the amplifier to reduce distortion. This arrangement does not require the precise setting of the adding and subtracting electronics disclosed by Hawksford and related circuits.
Audio power amplifiers usually consist of three definable stages: an input stage, voltage amplifier stage and output stage. Sometimes, the amplifier input stage and the voltage amplifier stage together are called the amplifier input stage. In power amplifiers, the output stage, sometimes called the power output stage, usually produces most distortion. However, the distortion of the power output stage may be substantially reduced by some of the concepts considered by me previously. Compared to these distortion reduced power output stages, the lowest distortion conventional input stages and voltage amplifier stages may produce substantially greater distortion. Conventional low distortion input stages are usually a differential voltage to current converter which produce a differential output current. In these low distortion traditional architectures, the differential current output of this input stage is connected to a current mirror, and the output node of the differential current output of the input stage and current mirror is connected to a common emitter cascode amplifier; the said common emitter amplifier sometimes being a Darlington. The amplifier""s dominant pole is set by a network including a capacitor connected between the output and input of this common emitter cascode stage.
In his second edition, Douglas Self disclosed the advantages of a second order global dominant pole, consisting of splitting the integrating capacitor in the voltage amplification stage, that is the said dominant pole setting capacitor, and connecting a resistor between ground and the said common split capacitor node. This allows for more overall global feedback, and thus reduced distortion. However, this adversely affects the amplifier slew rate owing to lower loading impedance on the output of the voltage amplification stage.
Linear Technology describes in application note AN67 a xe2x80x9csuper gain blockxe2x80x9d small signal amplifier consisting of effectively a 5th order global dominant pole. This is claimed to have an open loop gain of 180 dB at 10 kHz.
An object of this invention is to provide improvements which assist in even more accurate amplification or at least, provides the public with a useful alternative. This has particular application to audio power amplifiers, herein defined to produce at least 5W into 8 ohms at least at audio frequencies.
In one form of this invention this can be said to reside in an electronic amplifier having an input, and an output, and including an output stage containing output transistors being connected to the electronic amplifier output, the electronic amplifier input being connected to an input stage, an output of the input stage being connected to an input of the output stage, wherein a global dominant pole is formed which, not taking into account effects of any output stage local dominant pole, is at least of third order, at least at audio frequencies and lower ultrasonic frequencies.
In preference the electronic amplifier includes within the input stage, at least two amplifiers, a first and second amplifier, wherein the electronic amplifier input is connected to an input of the first amplifier, and an output of the first amplifier is connected to an input of the second amplifier, and an output of the second amplifier is connected to an input of the output stage, wherein there are at least two local negative feedback paths, a first and second local negative feedback path, a first local negative path being between an output of the first amplifier and an input of the first amplifier, a second local negative path being between an output of the second amplifier and an input of the second amplifier, and an overall negative feedback path is connected between an input of the first amplifier and the output stage, wherein there is at least a third order global dominant pole, at least at audio frequencies, when effects of any output stage local dominant pole are not taken into account.
In preference, a first local negative feedback path forms at least a local dominant pole about the first amplifier, a first local dominant pole, and the second local negative path forms at least a local dominant pole about the second amplifier, a second local dominant pole, and the said first local dominant pole is at least first order and the said second local dominant pole is at least second order, at least at audio frequencies.
In preference, in the alternative, the said second local dominant pole is at least first order and the said first local dominant pole is at least second order, at least at audio frequencies.
In preference, the said second amplifier consists of two series connected amplifiers, a third and fourth amplifier, and the said second local negative feedback path is connected between an output of the fourth amplifier and the input of the said third amplifier, and a third local negative feedback path is connected between an output of the third amplifier and an input of the third amplifier.
In preference, the output stage includes an output error correction stage containing at least one amplifier, a fifth amplifier, an input to the output stage being connected to an input of the fifth amplifier, wherein there are at least two local negative feedback paths, a fifth and sixth local negative feedback path, a fifth local negative feedback path being between an output of the output stage and an input of the fifth amplifier and a sixth local negative path being between an output of the fifth amplifier and an input of the fifth amplifier, an output of the fifth amplifier is connected to an input of output stage transistor buffers or the output stage transistors, an output of output stage transistor buffers, if used, being connected to an input of the output transistors, wherein the circuit arrangement and values of the said fifth and sixth local negative feedback paths and fifth amplifier and output transistors and output transistor buffers are selected to contain at least a first order local dominant pole, a third local dominant pole, at least at audio frequencies.
In preference, at least one of the said first, second, third or fifth amplifier is a wideband differential operational amplifier with a gain-bandwidth product of greater than 100 MHz and direct current open loop differential voltage gain of more than 200V/V.
In preference, the fifth amplifier is supplied by power from a floating power supply means coupled to an output of the output stage so that a voltage of the floating power supply supplying the fifth amplifier will follow substantially an output voltage of the output stage when operational.
In preference, the said third local dominant pole is at least second order.
In preference, the electronic amplifier is capable of delivering at least 5 Watts output into 8 ohms at least at audio frequencies.
An advantage of the invention lies in the discovery that high order global dominant poles may also be implemented in audio power amplifiers, and that this may quite easily be implemented with the use of operational amplifiers, and this high order dominant pole may be distributed across both the voltage amplification stage and input stage, without adverse reduction in slew rate.
This allows for considerably more negative feedback at audio and ultrasonic frequencies, thereby enabling considerable reduction in distortion across the entire audio band and some of lower ultrasonic band.
Further aspects of the invention including the scope of the invention can be gained by reference to the following description and the claims.
For a better understanding of this invention it will now be described with reference to a preferred embodiment which is described hereinafter with reference to drawings as follows